1. Field of the Invention
The disclosures herein generally relate to a crystal manufacturing apparatus, and more specifically to a crystal manufacturing apparatus for manufacturing the group III nitride crystals.
2. Description of the Related Art
InGaAlN (a group III nitride semiconductor) related devices, used for light sources such as ultraviolet, violet, blue, and green, are fabricated mostly on sapphire or silicon carbide (SiC) substrates using a MOCVD (Metal Organic Chemical Vapor Deposition) method or a MBE (Molecular Beam Epitaxy) method and the like.
When a sapphire or a SiC substrate is used as a substrate for fabricating a group III nitride semiconductor, a large number of crystal defects are introduced into the group III nitride semiconductor because of large differences in the lattice constant and the thermal expansion coefficient between those substrates and a group III nitride semiconductor. The crystal defects degrade the device characteristics. For an example, the crystal defects shorten the device lifetime and cause a larger current operation and the like, which relate directly to disadvantages of the devices.
Further, a sapphire substrate is an insulating material and it does not allow forming electrodes on it unlike a conventional substrate. This requires forming electrodes on the group III nitride semiconductor; as a result the area of a device becomes larger, which raises its manufacturing cost. As the device area becomes larger, there is a different problem arising as a wafer bending due to a combination of hetero-materials of a sapphire substrate and a group III nitride semiconductor.
Further, the group III nitride semiconductor device fabricated on a sapphire substrate is difficult to cleave, so that forming mirror facets to provide a resonant cavity of a laser diode (LD) is not facilitated. Thereby, presently, such resonant cavity facets are formed by using a dry etching technique or thinning the sapphire substrate to be less than 100 μm thick with a polishing technique and scribing it for easy cleaving. Thus, there is a difficulty forming resonant cavity facets and cleaving chips in a single process unlike a conventional LD fabrication, which causes a high production cost due to complicated fabrication processes.
Then, a technique is proposed where a group III nitride semiconductor is selectively, laterally grown on a sapphire substrate for reducing crystal defects. This allows reducing crystal defects; however, the problems due to non-conductivity of the sapphire substrate and a difficulty of cleaving still remain.
To solve such problems, a gallium nitride (GaN) substrate, the same material as a crystal to be grown, is most preferable as a substrate. For this reason, crystal growth techniques, such as gas phase growth and liquid phase growth are proposed for growing a bulk GaN crystal. However, a high quality GaN substrate with a large diameter for practical use has not been achieved.
As a method for obtaining a GaN substrate, a method of GaN crystal growth using sodium (Na) flux is proposed (see reference 1). This method utilizes sodium azide (NaN3) and metallic Ga as source materials that are placed in a reaction vessel (internal diameter: 7.5 mm, length: 100 mm) made of a stainless steel that is sealed with nitrogen atmosphere; the reaction vessel is heated to a temperature ranging 600° C.-800° C., and the temperature is maintained for a time period of 24-100 hours so that and then a GaN crystal is grown.
This method allows growing a crystal at a relatively low temperature 600° C.-800° C. with a relatively low pressure around 100 kg/cm2 in the vessel. It may be characterized as a practical crystal growth condition.
Further, recently a high quality group III nitride crystal has been achieved by reacting a mixed molten liquid of an alkali metal and a group III metal with a group V source material containing nitrogen (see reference 2).
However, as a conventional flux method performs crystal growth under a pressure ranging from several MPa to 10 MPa, a double wall vessel is used for separating functions of pressure resistance and heat resistance, and then a larger size inner vessel is developed for growing a larger size crystal (e.g. reference 2). In the case of a conventional flux method, since alkali metals are used, a glove box controlling the moisture and oxygen concentration to be less than 1 ppm needs to be used for supplying the source materials of a group III metal and an alkali metal into the inner vessel in the glove box. Thereby, the inner vessel is taken out from the outer vessel after every crystal growth cycle. During this time period the outer side of the inner vessel, the outer vessel, and the parts between the outer vessel and the inner vessel are exposed to air. Thus heaters and thermal insulators as well any parts located inside the outer vessel are exposed to air. Further, part of a nitrogen gas introducing tube in the inner vessel is also exposed to air. Over the region exposed to air, moisture and oxygen are absorbed and desorbed when raising temperature of the region or introducing nitrogen gas into the region. As a result, the heaters and the thermal insulators are degraded, and their impurities are contained in the source materials so as to affect crystal growth. Further, in terms of detaching the inner vessel and parts associated with it, the reproducibility of thermal distribution becomes a problem. In addition, detaching and attaching the inner vessel or the parts cause time loss which increases the production cost.
To avoid such problems, reference 3 suggests connecting an atmosphere control part to the space between a source weighting part and an outer vessel of a crystal growth apparatus to prevent mixing the air.
Reference 4 suggests supplying alkali metal and a group III metal into a crucible in a inner vessel.
Reference 1 U.S. Pat. No. 5,868,837
Reference 2 Japanese Patent Application Publication No. 2003-313099
Reference 3 Japanese Patent Application Publication No. 2005-335983
Reference 4 Japanese Patent Application Publication No. 2004-292286
In reference 3, however, the outer vessel is a pressure resident vessel, and it is difficult to provide a gate valve that connects to the atmosphere control part and allows source materials to pass through the gate valve at the same time.
Reference 4 shows that there is a need to open an inner vessel to take out a grown crystal while there is no need to open the inner vessel when a source material is supplied.
What is needed is a crystal manufacturing apparatus that grows a group III nitride crystal without exposing parts inside of the outer vessel to air while avoiding detachment of an inner vessel.